switch to eslint

[Josh-Schifter]

Enable eslint for frontend-sample-showcase

For the most part, changes to conform code to the default eslint rules from @bentley/build-tools were simple and make this code better. The following notes point out situations where I made some compromises:

• I disabled the rule deprecation/deprecation
  • eslint incorrectly believes document.createElement (and others) is deprecated
  • Seems to be a bug: document.createElement always marked deprecated microsoft/TypeScript#39212
• I fixed lots of unbound statics based on the @typescript-eslint/unbound-method rule
  • Ex. setup: HeatmapDecoratorApp.setup.bind(HeatmapDecoratorApp),
  • In hindsight, I think this is not an improvement. Readability seems worse.
  • Also seems unintended since since ignoreStatic is set in @bentley/build-tools
• I expanded allowed function naming via @typescript-eslint/naming-convention
  • this is only for tsx file
  • the default rule conflicts with the usual convention for react function components
  • hoping to hear back from UI devs about a better approach
• I setup ignore for all .d.ts files.
  • If I don't ignore I get this error
    • Parsing error: "parserOptions.project" has been set for @typescript-eslint/parser.
  • Seems ok since these are generated